Modulating system for short waves



I. WOLFF MODULATING SYSTEM FOR SHORT WAVES April 27, 1937.

Filed Aug. 51, 1955 2 Sheets-Sheet l Wag Patented Apr. 27, 1937 PATENT OFFIQE 2,078,302 MODULATTNG SYSTEM FOR SHORT WAVES Irving Wolff, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 31,

8 Claims.

vMy invention relates to radio apparatus and particularly to means for and methods of modulating or controlling radio energy having a short Wa l I While there are" many advantages in the use of such radio energy, ithas been found very diflicult to modulate it to the desired degree without changingits wave length, that is, usually both amplitude and frequency modulation are obtained instead of a pure amplitude modulation.

I have discovered that this 'difficulty can be overcome by intercepting the path of the radio waves by means of a device which is electrically independent of the high frequency generator and by. varying the electrical or mechanical characteristics, or both, of this device in accordance with a signal. H I f In the case of radio energy having an extremely shortwave length, such as a wave length of the order of 10 centimeters, for example, and at present involving great difficultyin modulating the energy, iii-accordance with the invention I preferably concentrate the radio energy into the form of a beam and intercept this beam bya modulating device. I

From the foregoing it will be apparent that an object of my invention is to provide an improved method and means for modulating high frequency radio energy in a system of the above-mentioned ype- More specifically, an object of my invention is to provide a method and means for modulating radio energy at the very short Wave lengths with out producing frequency modulation.

A still further object of my invention is to provide a modulating device for a beam of radio energy which will produce no reaction upon the high frequency generator.

Other features and advantages of my invention wil appear from the following description taken in connection with the accompanying drawings in which---.

Figure 1 is a perspective view of an embodiment of-my invention utilizing a mechanical shutter;

Fig. 2 is a schematic view showing a modified form of the apparatus illustrated in Fig. 1;

Fig. 3 is a perspective view illustrating another embodiment of my invention;

' Fig. 4 is a view illustrating the operation of the resistance varying units shown in Fig. 3;

Fig. 5 is a perspective view of a modified form of the apparatus shown in Fig. 3;

Fig. 6 illustratesv a modified arrangement of the apparatus shown in Figs. 3 and 4;

Fig. 7 is a. view showing another embodiment 1933, Serial No. 687,599

of my invention in which the plane of polarization of the radio beam is utilized;

Fig. 8 is aview showing an optical system which may be employed with any of the apparatus shown in the preceding figures;

Figs. 9 and 10 are views showing modified forms of my invention; and

Fig. 11 is a view showing a vacuum tube circuit which may be substituted, for the device shownin Fig. 4. r

The embodiment of my invention illustrated in Fig. 1 includes a radio beam transmitter which may be of any of the well known. constructions. The transmitter illustrated comprises a parabolic reflector I having a dipole antenna 3 positioned therein at the principal focus of the reflector. The antenna 3 is connectedto a high frequency generator 5 by means of the parallel conductors 1 which also serve as supports for the antenna 3. In place of the dipole antenna and reflector, any directional antenna, such as an antenna array, may be employed.

The high frequency generator 5 may be of any type which will have a suitable power output. The well known magnetron oscillator has been found to be satisfactory.

The antenna 3 is preferably one half wave length long. Thus, if a radio beam having a wave length of approximately 9 centimeters is to be transmitted, the dipole antenna may have a length of approximately 4 centimeters, it being necessary to allow for certain end efiectsr The supporting conductors 1 in this case are connected to the antenna 3 at points approximately 8 millimeters apart.

Instead of followingv past practice and attempting to put an amplitude modulation on the radio beam by means of a modulating circuit connected to the high frequency generator, I intercept the radio beam by means of a modulating device 9 which is electrically independent of the high frequency generator 5. The modulating device 9 shown in Fig. 1 comprises a plurality of metal shutter vanes ll pivotally mounted upon a base l3. The vanes I I are pivotally secured to an operating arm l5 which is connected to the driving coil ll of a loud speaker unit 19. The loud speaker unit [9 is preferably of the dynamic type and may be of any well known form of construction.

While the spacing between shutter vanes ll is not critical, they are preferably spaced apart of the vanes H are too close together, the shutter, even while open, will reflect a large percentage of the radio beam.

For the purpose of explanation it will be assumed that when the vanes II are closed, they are in their zero position, and that when they are completely open as illustrated in Fig. 1, they are in their 90 degree position. For the point of zero modulation, the vanes will be set at some point intermediate the zero and 90 degree positions. For example, they may be set in the 60 degree position and varied about this position by means of voice currents or the like passed through the driving coil ll of the loud speaker unit I9. The amount of energy passing through the shutter 9 will. depend upon the opening of the shutter, that is, upon the effective spacing between vanes, a greater amount of energy passing through the shutter as the vanes approach the 90 degree position.

The modulated radio beam may be received by any of the well known short wave receivers. The receiver illustrated comprises a parabolic reflector 23 having a dipole antenna 25 positioned therein and supported by conductors 21 which connect the antenna, 25 to any suitable form of receiver 29.

It will be apparent that since the above-described form of modulator is independent of the high frequency generator 5 it will produce little, if any, variation in the frequency of the generator output. The only variation in frequency which it may produce will be that caused by a certain amount of energy being reflected back into the transmitter reflector where the load on the antenna 3 may be changed slightly, but this effect can be made negligible by locating the shutter at a, distance away from reflector L It will also be apparent that with the above-described arrangement, the generator 5 may provide maximum output without regard to operating on any particular point of the generator characteristic for obtaining proper modulation.

Any difficulty which might be caused by energy reflected back from the modulating device 9 may be avoided by the arrangement shown in Fig. 2. Here a shutter 3| is set at an angle of 45 degrees with respect to the axis of the radio beam. With the shutter 3| so located, any part of the radio beam reflected from the shutter vanes 33 will be reflected in such a direction that it does not enter the parabolic reflector For example, when the shutter 3| is completely closed, the vanes 33 will be set at an angle of 45 degrees with respect to the axis of the beam, and the parallel rays emerging from the parabolic reflector I will be reflected from the vanes 33 in an upward direction.

In the arrangement of Fig. 2 the shutter 3| is operated by means of a solenoid 35 connected to a keying circuit, the solenoid winding 31 being connected in series with a battery 39 and the key 4|. The shutter 3| is held in the open position indicated in Fig. 2 by means ofa spring 43. If desired, the driving unit |9 shown in Fig. 1 may be substituted for the keying'circuit of Fig. 2. Also, the keying circuit may be used with the apparatus of Fig. 1, if desired.

In Fig. 3 there is illustrated a modulating device 44 which is electrically, instead of mechanically, operated. It comprises a group of parallel conductors 45 located in front of the parabolic reflector of the transmitter so that they scatter, reflect, or absorb a portion of the radio beam striking it. Preferably the conductors 45 of the modulating device 44 are spaced apart a distance less than one wave length of the radio beam. It is well known that if the conductors in such a reflecting arrangement have good conductivity, a fairly large percentage of a radio beam which strikes them will be reflected, and, also, that a certain percentage of the beam will be scattered or diverted from its original path.

In accordance with my invention, I place resistance varying devices 41 in these conductors to vary their conductivity. A resistance varying device 41 is placed in the middle of each conductor 45 where, by making the conductors 45 an odd number of half wave lengths long, there is maximum current flow.

The resistance varying units are connected together so that they may be operated simultaneously by means of the modulating current. In Fig. 3 they are shown connected in parallel to the output of an audio frequency amplifier 49 by means of conductors 5|, the input of the amplifier 49 being connected to a microphone 53.

Various forms of resistance varying devices may be employed for varying the conductivity of the conductors 45. One such device is illustrated in Fig. 4, and comprises a carbon button positioned between the supporting element 51 and the armature 59 of an electromagnet 6|. The electromagnet coil 63 is so connected to the output of the audio frequency amplifier 49, as indicated in Fig. 3, that the direct current component of the plate current passes through the coil 63 to hold the armature 59 firmly against the carbon button 55. Variations in plate current will move the armature 59 and thus vary the conductivity of the carbon button 55 in accordance with such variations.

It is well known that the radio beam transmitter of the type illustrated radiates a beam which is strongly polarized, the plane of polarization of the electric field being in the plane of the dipole antenna 3. For this reason, the conductors 45 of the modulating screen 44should be positioned approximately parallel to this plane of polarization in order to make the screen operative. If the conductors 45 were placed perpendicular to the plane of polarization, they would have substantially no effect upon the radio beam.

In Fig. 5 there is shown a modified form of my modulating screen. In this modified form, a plurality of conductors 65 are each tuned to the Wave length of the radio beam. Instead of employing one conductor with a resistance varying device in the middle, it is replaced by a plurality of conductors 65 one half wave length long. Each half wave length conductor 65 has a resistance varying device 61 connected in the middle for varying the conductivity of the conductor.

The resistance varying devices 61 may be constructed as shown in Fig. 4 and may be connected in any suitable manner to be operated simultaneously, such for example as in the manner illustrated in Fig. 3. By utilizing'a plurality of tuned conductors 65, the efficiency of the modulating screen is considerably greater than in an arrangement where untuned conductors are employed. For the reasons given above, the conductors 65 should be placed parallel to the plane of polarization (the electric field) of the radio beam.

As explained in connection with the mechanical shutter, it is desirable to prevent the energy reflected from the modulating device from entering the parabolic reflector of the transmitter. This may be avoided by placing either the modulating screen 'shown'in Fig.3,or the one shown in Fig. 5, at an angle with respect to the axis'of the radio beam, as illustrated in Fig. 6. Preferably, the screen is set at an angle of 45 degrees, although it may be set at a differentangle if desired. I

In adjusting the above-described modulatin screens for best operation, it will usually be found desirable to make the unmodulated resistance of the variable resistance device equal to the radiation resistance of the screen radiation element in which it is connected. Thus, in the apparatus shown in Fig. 3 the unmodulated resistance of a device 41 would be made equal to the radiation resistance of an element 45.

During modulation, the resistance of a device 4! will vary from a high value of resistance to a low value of resistance When the resistance is very high, the modulatingscreen will have but little effect upon the radio beam. As the resistanceis'decreased, the screen absorbs more energy ('atthe same time reflecting and scattering some energy) until the maximum amount is absorbed at the above-mentioned zero modulation adjustment. A still further decrease in the resistance causes the absorbing action to decrease, but it also causes the reflecting and scattering action to increase so that there is a decrease'in the amount of energy reaching the receiver.

- In the above paragraphs, the term scattering is used to describe the dispersion or diffraction effect which causes some of the radio beam to be diverted or spread out as it leaves the modulating screen. The term reflection is applied to any effect which causes some of the radio energy striking the screen to be sent back therefrom."

While it has been stated that the radiating elements of my modulating screen should be less than one wave length apart, equally effective modulation may be obtained by using a wider spacing of elements and employing two or more modulating screens. For example, it will be seen that a plurality of rows of modulating devices as in Fig; 5 may be arranged in any other spaced relation to each other, such as spaced along instead of intercepting the axis of the radio beam. With such an arrangement, any energy passing between two elements of one screen will strike an element of a second screen.

In Fig. 7 I have shown a modulating device 68 which takes advantage of the fact that the radio beam is polarized. It comprises a plurality of conductors 69, which preferably are one half wave length long, pivoted at their middle. Three rows of these conductors are illustrated, the conductors in each row being pivotally connected to an operating arm II. The three operating arms H are connected to driving device 13 which may be an electromechanical device like that shown in Fig. 1 at I9. This is arranged for tilting the conductors 99 about their pivot points at an angle to the vertical plane.

In connection with the modulating means, a dipole antenna such as 3 positioned in the parabolic reflector I is set so that the plane of polarization of the electric field is in the vertical plane. It is apparent, therefore, that when the half wave length conductors 69 are parallel to the vertical plane they form a good reflector for the radio beam. If they are moved into a horizontal position by the driving unit 13 they have substantially no reflecting action. Between these two limits of operation they have a varying reflecting ability depending upon the amount they have been tilted out of the vertical plane by the driving unit 13. Consequently the radio beam which passes through the modulating reflector 68 is modulated in accordance with the current which operates the driving unit 13.

Since it is impossible to produce a well defined radio beam by means of a small reflector, an arrangement such as the one indicated in Fig. 8 may be desirable. In this arrangement the parabolic reflector 75 is made as large as necessary in order to avoid diffraction effects, and then the beam is concentrated by means of a lens 11. A comparatively small shutter or modulating reflector 19 may then be positioned in the radio beam at a point where-the beam has a comparatively small cross-section, and then the rays of the beam may again be made parallel by means of a second lens 3i. This will make it possible to employ a mechanical shutter having moving parts with small inertia.

While it is likely that my invention will be found most useful in modulating energy concentrated in the form of a beam, it is not limited to beam transmission. As indicated in Fig. 9, radio energy may be radiated from an antenna 83 in all directions, and modulated by means of a modulating screen 85 similar to the one shown in Fig. 3 (or Fig. 5, if preferred), but surrounding the antenna. As explained in connection with Figs. 3 and 5, the resistance varying devices 81' are placed in conductors 89 and so connected that they may be operated simultaneously. For

the reason that suitable connections and means 2/ for that purpose are shown in Figs. 3 and 4, for example, such connections are not repeated in the present figure.

If desired, a modulating screen 9| with its variable impedance conductors 93 may be shaped in the form of a parabola as indicated in Fig. 10, and utilized both to concentrate and modulate the radio energy. The antenna 95 is placed at the principal focus of the parabolic reflector 9!.

Fig. '11 shows an electron discharge amplifier circuit or system which may be substituted for the variable impedance modulating device previously described. In the example illustrated, each variable impedance device 98 comprises a three electrode electronic tube 91 having a plate 99 supplied with positive potential through a choke coil I9l. One half of a vertical, divided modulating screen conductor I03 is connected at its lower end to the plate 99 while the other half of the screen conductor |03a is connected at its upper end to the cathode I05.

The plate impedance of a tube 91, and therefore the conductivity of the screen conductor I03, is varied by means of a negatively biased control grid I91 connected to the secondary of an audio frequency transformer I09. Several variable impedance devices 98 are connected in parallel as indicated in the drawing. While only two screen conductors I03 with their associated variable impedance units 98 have been shown, it will be understood that, in practice, several more screen conductors would be employed, as indicated in certain of the preceding figures.

It should be understood, however, that in its simplest form my modulating screen comprises a single conductor and associated variable impedance device. For example, in apparatus of the type shown in Fig. 10, it will be apparent that the energy radiated from the antenna 95 may be directed more strongly in one direction than in 93 at the rear of the antenna.

Various other modifications may be made in my invention without departing from the spirit and scope thereof, and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and are imposed by the appended claims.

I claim as my invention:

1. Electrical apparatus comprising means for generating and transmitting a beam of energy at a radio frequency, and means positioned at an angle with respect to said beam for absorbing said beam a variable amount in accordance with a signal.

,2. Electrical apparatus comprising means for generating energy at a high radio frequency, means for transmitting said energy in the form of a beam, and means including a variable resistance screen in the path of said beam and positioned at an angle with respect to said path for modulating said transmitted energy in accordance with a signal representing current.

3. In combination, means for generating and transmitting a beam of radio frequency energy, a scattering and reflecting device located in the path of said beam and in line with said means and a receiver, said device being positioned with its reflecting surface at an acute angle with respect to the axis of said beam, and means for varying the scattering and reflecting ability of said device in accordance with a signal.

4. In combination, means for generating energy at a high radio frequency, means for radiating said energy in the form of a beam, a device located in the path of said beam which partially reflects said energy, said device being placed in line with the second means and a receiver at such an angle with respect to the axis of said beam that the amount of reflected energy reaching said second means is small, and means for varying the reflecting ability of said device in accordance with a signal.

5. In combination, means for generating energy at a radio frequency, means for radiating said energy, a variable impedance screen positioned another by utilizing a single one of the conductors at an angle. with respect to the wave front of said radiated energy to intercept said radiated energy, said screen comprising a plurality of conductors, each conductor having a variable impedance device therein, and means for varying the impedance of said devices in accordance with a signal.

6. In combination, means for generating energy at a radio frequency, means for radiating and directing said energy in a certain direction, and a modulating device positioned at an angle with respect to the wave front of said radiatedenergy to intercept said radiated energy, said device comprising a plurality of spaced parallel conductors, each of said conductors having a variable impedance unit connected in series therewith substantially midway between the ends thereof, and means for varying the impedance of said units in accordance with a signal.

7. In combination, means for generating energy at a radio frequency, means for radiating and directing said energy in a certain direction, and a modulating device positioned at an acute angle with respect to the wave front of said radiated energy to intercept said radiated energy, said device comprising a plurality of parallel con-- ductors spaced apart less than one wave length of said radiated energy, each of said conductors having a variable impedance unit connected in series therewith at approximately midway between the ends of the conductor, and means for varying the impedance of said units in accordance with a signal.

8. In combination, means for generating energy at a radio frequency, means for radiating and. directing said energy in a certain direction, and. a modulating device positioned at an acute angle with respect to the wave front of said radiated energy to intercept said radiated energy, said device comprising a plurality of spaced parallel conductors, each of said conductors having a variable impedance unit connected in series therewith at approximately midway between the ends of the conductor, and each conductor being tuned to the wave length of said radiated energy.

IRVING WOLFF. 

